Ruby 3.5 moves rdoc as bundled gem

This commit addresses the following error:
```ruby
% ruby -v
ruby 3.5.0dev (2025-03-11T00:00:47Z master 47c3ae6962) +PRISM [arm64-darwin24]
~/src/github.com/rails/thor %
% bundle exec rspec ./spec/actions/create_link_spec.rb
/Users/yahonda/src/github.com/rails/thor/spec/helper.rb:18: warning: rdoc used to be loaded from the standard library, but is not part of the default gems since Ruby 3.5.0.
You can add rdoc to your Gemfile or gemspec to fix this error.

An error occurred while loading ./spec/actions/create_link_spec.rb. - Did you mean?
                    rspec ./spec/actions/create_file_spec.rb

Failure/Error: require "rdoc"

LoadError:
  cannot load such file -- rdoc
No examples found.

Finished in 0.00002 seconds (files took 0.11832 seconds to load)
0 examples, 0 failures, 1 error occurred outside of examples

Coverage report generated for RSpec to /Users/yahonda/src/github.com/rails/thor/coverage.
Line Coverage: 34.9% (511 / 1464)
[Coveralls] Outside the CI environment, not sending data.
Stopped processing SimpleCov as a previous error not related to SimpleCov has been detected
%
```

Refer to:
ruby/ruby#12577
https://bugs.ruby-lang.org/issues/20309

Author: yahonda

Gemfile

@@ -15,6 +15,7 @@ group :test do
   gem "rspec-mocks", ">= 3"
   gem "simplecov", ">= 0.13"
   gem "webmock", ">= 3.14"
+  gem "rdoc"
 end
 
 gemspec